The present invention relates to the general field of controlling variable-pitch vanes. A particular application lies in the field of aviation, and in particular to controlling the angular positions of vanes directing air flow into turbomachine compressors.
Known devices for controlling variable-pitch vanes in a turbomachine usually comprise a control member in the form of a ring surrounding a casing of the turbomachine and a plurality of levers or rods, each rod having a first end connected to the control ring via a hinge and a second end mounted on the pivot of a respective vane. Synchronized modification of the angular position of the vanes is achieved by turning the ring around the axis of the turbomachine.
In order to be able to follow the turning movement of the ring, the connection between each rod and the ring includes at least one degree of freedom in rotation about an axis extending substantially radially relative to the ring. Nevertheless, since the rod is rigidly mounted on the corresponding vane pivot, turning the ring leads to other relative movements between the ring and the portion of the rod mounted on the vane pivot.
In order to accommodate these additional movements, or at least some of them, it is well known to make the connection in the form of a ball joint or the like. The mechanical hinge between the vane pivot and the end of the rod mounted thereon can be achieved in various ways. For example, the vane pivot may have a threaded end passing through an orifice pierced in the control rod, a nut tightened onto the threaded end of the pivot serving to cause these two parts to turn together once their angular position has been set.
Thus, hinges are known in which the control rod is driven using a drive square without taking up slack. In that prior art, the lack of accuracy in control rod turning relative to the vane pivot lies in the range 0.5° to 0.7° (because of manufacturing tolerances). This lack of accuracy in positioning the rod is particularly harmful to good operation of the assembly as a whole).